Resolve "buffer too small" error with some bigger certs

This patch allows to use PPTP VPN from Linux to Windows Server using Aladdin eToken PRO 64K. Without the patch, for some certificates pppd will fail with error: "pkcs11_init_cert: buffer too small" and refuse to establish connection.

This fixes ticket #350 in libp11 trac:
https://www.opensc-project.org/opensc/ticket/350